Smartgwt Typeerror : Cannot Read Property of Null

JavaScript Errors and How to Fix Them

JavaScript tin can be a nightmare to debug: Some errors it gives can exist very difficult to understand at start, and the line numbers given aren't ever helpful either. Wouldn't information technology be useful to have a list where you lot could look to detect out what they mean and how to gear up them? Here you go!

Below is a list of the strange errors in JavaScript. Different browsers can give you different messages for the same fault, so there are several dissimilar examples where applicable.

How to read errors?

Earlier the list, let'southward quickly look at the structure of an error message. Understanding the structure helps understand the errors, and you'll have less trouble if you run into any errors not listed here.

A typical error from Chrome looks like this:

Uncaught TypeError: undefined is not a function

The construction of the mistake is as follows:

1. Uncaught TypeError: This office of the bulletin is ordinarily not very useful. Uncaught means the error was not caught in a catch statement, and TypeError is the fault'south name.
2. undefined is not a role: This is the message part. With fault letters, y'all have to read them very literally. For example in this case it literally means that the code attempted to use undefined similar information technology was a function.

Other webkit-based browsers, like Safari, requite errors in a similar format to Chrome. Errors from Firefox are similar, but exercise not always include the commencement part, and recent versions of Cyberspace Explorer besides requite simpler errors than Chrome – but in this instance, simpler does not ever hateful better.

At present onto the actual errors.

Uncaught TypeError: undefined is not a part

Related errors: number is not a office, object is non a role, cord is not a function, Unhandled Error: 'foo' is not a role, Role Expected

Occurs when attempting to phone call a value like a function, where the value is not a function. For example:

var foo = undefined; foo();

This mistake typically occurs if yous are trying to phone call a function in an object, but you typed the proper noun wrong.

var x = certificate.getElementByID('foo');

Since object properties that don't exist are undefined by default, the above would result in this error.

The other variations such as "number is not a role" occur when attempting to telephone call a number like it was a function.

How to ready this fault: Ensure the office name is right. With this error, the line number will normally point at the right location.

Uncaught ReferenceError: Invalid left-hand side in assignment

Related errors: Uncaught exception: ReferenceError: Cannot assign to 'functionCall()', Uncaught exception: ReferenceError: Cannot assign to 'this'

Acquired by attempting to assign a value to something that cannot exist assigned to.

The most mutual case of this error is with if-clauses:

if(doSomething() = 'somevalue')

In this example, the programmer accidentally used a unmarried equals instead of 2. The bulletin "left-hand side in assignment" is referring to the role on the left side of the equals sign, so like y'all can meet in the above example, the left-hand side contains something you tin can't assign to, leading to the error.

How to fix this error: Make certain yous're not attempting to assign values to part results or to the this keyword.

Uncaught TypeError: Converting round construction to JSON

Related errors: Uncaught exception: TypeError: JSON.stringify: Non an acyclic Object, TypeError: cyclic object value, Circular reference in value argument not supported

Always caused by a circular reference in an object, which is then passed into JSON.stringify.

var a = { }; var b = { a: a }; a.b = b; JSON.stringify(a);

Considering both a and b in the in a higher place example have a reference to each other, the resulting object cannot exist converted into JSON.

How to fix this error: Remove circular references like in the example from any objects you want to catechumen into JSON.

Unexpected token ;

Related errors: Expected ), missing ) after argument list

The JavaScript interpreter expected something, but it wasn't there. Typically acquired by mismatched parentheses or brackets.

The token in this fault can vary – it might say "Unexpected token ]" or "Expected {" etc.

How to fix this fault: Sometimes the line number with this error doesn't betoken to the correct place, making it hard to set up.

• An mistake with [ ] { } ( ) is usually caused by a mismatching pair. Check that all your parentheses and brackets have a matching pair. In this case, line number will often point to something else than the trouble character
• Unexpected / is related to regular expressions. The line number for this volition ordinarily be correct.
• Unexpected ; is normally caused past having a ; inside an object or array literal, or within the argument list of a function call. The line number will usually be right for this case also

Uncaught SyntaxError: Unexpected token ILLEGAL

Related errors: Unterminated String Literal, Invalid Line Terminator

A string literal is missing the closing quote.

How to fix this fault: Ensure all strings take the correct closing quote.

Uncaught TypeError: Cannot read property 'foo' of cipher, Uncaught TypeError: Cannot read property 'foo' of undefined

Related errors: TypeError: someVal is null, Unable to go property 'foo' of undefined or zippo reference

Attempting to read zip or undefined as if information technology was an object. For example:

var someVal = naught; panel.log(someVal.foo);

How to fix this error: Usually caused by typos. Cheque that the variables used near the line number pointed by the error are correctly named.

Uncaught TypeError: Cannot set property 'foo' of nada, Uncaught TypeError: Cannot set up belongings 'foo' of undefined

Related errors: TypeError: someVal is undefined, Unable to prepare property 'foo' of undefined or null reference

Attempting to write zilch or undefined as if it was an object. For example:

var someVal = naught; someVal.foo = 1;

How to gear up this error: This likewise is usually caused past typos. Check the variable names about the line the error points to.

Uncaught RangeError: Maximum call stack size exceeded

Related errors: Uncaught exception: RangeError: Maximum recursion depth exceeded, likewise much recursion, Stack overflow

Usually caused past a bug in program logic, causing infinite recursive role calls.

How to fix this fault: Check recursive functions for bugs that could crusade them to keep recursing forever.

Uncaught URIError: URI malformed

Related errors: URIError: malformed URI sequence

Caused by an invalid decodeURIComponent call.

How to gear up this error: Bank check that the decodeURIComponent call at the error's line number gets correct input.

XMLHttpRequest cannot load http://some/url/. No 'Access-Control-Allow-Origin' header is present on the requested resources

Related errors: Cantankerous-Origin Request Blocked: The Same Origin Policy disallows reading the remote resources at http://some/url/

This error is always caused by the usage of XMLHttpRequest.

How to fix this error: Ensure the request URL is correct and it respects the same-origin policy. A good fashion to detect the offending code is to look at the URL in the error message and find it from your code.

InvalidStateError: An attempt was made to employ an object that is not, or is no longer, usable

Related errors: InvalidStateError, DOMException lawmaking eleven

Means the code chosen a role that you should not call at the electric current land. Occurs usually with XMLHttpRequest, when attempting to call functions on it before it's ready.

var xhr = new XMLHttpRequest(); xhr.setRequestHeader('Some-Header', 'val');

In this case, you would go the error considering the setRequestHeader function can only be called after calling xhr.open up.

How to fix this fault: Await at the code on the line pointed by the error and make sure it runs at the correct time, or add any necessary calls earlier it (such every bit xhr.open up)

Decision

JavaScript has some of the most unhelpful errors I've seen, with the exception of the notorious Expected T_PAAMAYIM_NEKUDOTAYIM in PHP. With more familiarity the errors outset to make more than sense. Modern browsers also assist, every bit they no longer give the completely useless errors they used to.

What's the near disruptive fault y'all've seen? Share the frustration in the comments!

Want to acquire more about these errors and how to prevent them? Detect Problems in JavaScript Automatically with ESLint.

Virtually Jani Hartikainen

Jani Hartikainen has spent over 10 years building web applications. His clients include companies like Nokia and hot super secret startups. When not programming or playing games, Jani writes nigh JavaScript and loftier quality code on his site.

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Source: https://davidwalsh.name/fix-javascript-errors

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